Conjugated polyelectrolytes (CPEs) with π‐delocalized main backbones and ionic pendant groups are intensively studied as interfacial layers for efficient polymer‐based optoelectronic devices (POEDs) because they facilitate facile control of charge injection/extraction barriers. Here, a simple and effective method of performing precise interfacial energy level adjustment is presented by employing CPEs with different thicknesses and various ion densities under electric poling to realize efficient charge injection/extraction of POEDs. The effects of the CPE ion densities and electric (positive or negative) poling on the energy level tuning process are investigated by measuring the open‐circuit voltages and current densities of devices with the structure indium tin oxide/zinc oxide/CPE/organic active layer/molybdenum oxide/gold while changing the CPE film thickness. The performances of inverted polymer light‐emitting diodes and inverted polymer solar cells are remarkably improved by precisely controlling the interfacial energy level matching using optimum CPE conditions.